I use firebug for a while and I love it especially for CSS and JS debugging. Today I stumbled over Yslow and did some tests with it one some of my clients pages and I love the results.
YSlow can help you to analyze all the referenced elements on a webpage. It shows their sizes, how they behave when a page is taken from the browser-cache, shows optimization hints and enables a easy way to optimize the output of your dynamically created webpage :) … you’ll love it.
Get YSlow here :)